The present invention is intended for a complete and qualitative cleaning of casing string sections from any deposits and built-up metal, as well as for restoring a complete passability of separate well bore sections along the inside diameter. The arrangement is operated in drilling mud, water, oil and mineralized stratum fluid.
In addition the present invention may advantageously be used for cleaning tubular heat exchangers of scale (in the process of overhaul) and other tubular internal surfaces with a constant or slightly varying inside diameter.
The practical remedy of casing strings by means of steel patches proves that a positive result of this operation is possible only if the internal surface of the pipes is thoroughly cleaned along the string section wherein the patch it to be installed.
The presence of corrosion products, cement cake, deposits of salts, as well as burrs and built-up metal on the walls of a casing string in the zone of perforation excludes the snug bearing of the patch against the casing pipe and consequently the achievement of the tightness thereof. When the patch is expanded in a contaminated casing pipe, it may be displaced along the casing string.
The use of pressuring packers and packers to be installed on the tubing string in injection wells, as well as the use of other downhole equipment requiring the snug bearing against the pipe wall (formation testers, and so forth) or power cooperation therewith (anchors) are likewise hampered.
Known in the prior art is a mechanical scraper for cleaning casing strings (Composite Catalog, v. 1, 1984-1985, pp. 808-809, model "C-3", roto-vert, casing scraper, product 620-03), comprising a massive hollow body externally accommodating cutting blades constantly forced by springs against the wall of a casing string under cleaning. The scraper design contemplates the use of a plurality of sets of changeable cutting blades each of which is intended for operation in a narrow range of well bore inside diameters (depending on standard sizes of casing pipes and thickness of their walls).
The mechanical scraper is run in a well to a section to be cleaned on a drill pipe string or tubing string and is commonly operated in conjunction with recirculation of the washing fluid for carrying the products of cleaning away from the wall to the surface.
The process of cleaning may be accomplished both by rotating the scraper and feeding it simultaneously downwards, and by reciprocating it along the string.
The first method is more efficient, however, it is used preferably for cleaning casing pipes of soft and homogeneous deposits of salts, gypsum, mud cake, etc. However when a casing string is to be cleaned of a cement cake, it is recommended to rotate the scraper and simultaneously to drill out the cement bridging plug.
When cleaning the casing string of hard heterogeneous deposits, and moreover when dealing with burrs and built-up metals in the zone of perforation, rotation of the scraper leads to jerks and twisting of the string resulting in jamming and break of the pipes.
For cleaning in such conditions, it is recommended to use the reciprocating motion of the scraper.
A disadvantage of the heretofore described method resides mainly in a low efficiency of cleaning resulting from an inconsiderable force exerted by cylindrical or flat springs for pressing the cutting blades against the surface being cleaned. The force applied to one cutting blade in an appropriate range of the well bore inside diameters comprises 0.7-1.5 kN which corresponds to an average specific load of 0.1-0.2 MPa on the working surface and may ensure only a low-efficient surface attrition. A small working range of compression of the springs limits the radial travel of a cutting blade (up to 5 mm) which involves the use of a plurality of sets of cutting blades in each standard size of the scrapers.
The fact that the cutting blades of a mechnical scraper are constantly forced against the surface being cleaned involves difficulties in entering the scraper in a casing string, reduces the speed of the scraper running the well because of a possible sticking in contaminated sections of the string and in well bore sections with an excessive curvature. This causes an additional wear of cutting edges. In the casing string sections with cracks, burnouts and especially in the zone of perforation, the speed of the scraper running is limited down to 10 m/min, as despite inconsiderable force the cutting blades may thrust against hard projections or sink in the spaces therebetween, thereby causing sticking of the tool with hazardous consequences.
Cleaning the casing string of hard deposits requires prolonged multihour operation of the scrapers, however metal projections on the string surface remain quite unaffected by cleaning.
Also known in the prior art is a hydromechanical arrangement for cleaning uncased well bores of loose mud cake (SU, A No. 649,829).
The arrangement is provided with an elastic extending vessel accommodated in the body. While expanding under the action of a pressure differential, the vessel acts directly on pivoted segments (cutting blades), thereby extending them from the slots of the body until they come in contact with the surface to be cleaned. The pivoted segments in are installed in the body on pivot pins. In order to fasten the ends of the vessel, a bushing with ducts for passage of fluid is provided thereinside. The hydromechanical arrangement is designed for mud grouting of the walls of uncased well bore in the process of drilling. The arrangement has run-in and working positions. The arrangement is changed over from the run-in to the working position under the action of a pressure differential built up in the working space due to the slush nozzles of a bit after lowering in the well to the section to be cleaned.
The arrangement is operated only by means of rotary motion at a slight pressure differential which along with the action of the centrifugal force on the pivoted segments provides on the contacting surface a force sufficient for a partial cleaning and a loose mud cake grouting. The arrangement is designed for operation on the shaft of a turbodrill.
Disadvantages of this arrangement preventing its effective use for cleaning the casing strings reside in the following.
Effective cleaning of a casing string from built-up metal and dense deposits requires that the cutting blade is pressed against the casing pipe wall with a force of several tons to which corresponds operation of the scraper at a pressure differential of 4-6 MPa. When the cutting blade is extended into the working position the vessel walls are subjected to extension. A sharp bend and an additional local extension of the vessel elastic material are caused under the action of the pressure differential on the shoulders between the body and the extended cutting blade, especially in a longitudinal section of the scraper. Besides, at a great pressure differential the elastic material of the vessel is forced (flows) in the clearance between the body and the movable cutting blade and is further pinched (seized). Multiple extension on the shoulders and pinching of the elastic material leads to a rapid destruction of the vessel.
When casing strings are under overhaul the scraper is usually run in on the tubing allowing the cleaning to be carried out only by the reciprocating motion without rotating the scraper when fails to ensure the effective cleaning of the internal surface of the casing strings from hard deposits and built-up metal. The scraper with the pivoted cutting blades (segments) is not fit for operation in such conditions, as the cutting blades have a slight almost linear contact with the circumference of the surface being cleaned.
The arrangement is not provided with a mechanism for returning the pivoted segments to the run-in position after release of the pressure.